In industry, the use of metal powder products manufactured by compacting and sintering metal powder compositions is becoming increasingly widespread. A number of different products of varying shape and thickness are being produced, and the quality requirements placed on these products are constantly increasing.
There are several advantages with using powder metallurgical methods for producing structural parts compared to machining or casting. As net shape or near net shape components can be produced, the material utilisation is much higher compared to machining of a components from ingot or wrought steel, and the energy consumption is much lower compared to when producing components by casting.
In order to facilitate the compaction and ejection of the compacted component from the die, lubricants are added to the metal powder composition. The lubricant is intended to reduce the friction between the individual powder particles during the compaction step, promoting the possibility of reaching high green density as well as being able to form a lubricating layer between the surfaces of the component and the die during the ejection step and reducing the force needed in order to eject the component as well as prohibiting scoring or the formation of scratch marks on the surface of the ejected component. Furthermore, a good lubricant shall not negatively influence the powder properties, i.e. apparent density, AD, and flow. AD is a measure of the bulk density of the powder or the volume occupied by the powder composition after filling of the die, expressed as grams/cm3, and measured according to ISO 3923-1. Flow is a measure of how fast a fixed amount, 50 grams, of the powder composition can flow through a standardized funnel, measured in seconds. The method is described in ISO 4490. Normally a high value of AD is preferred allowing shorter punches and shorter ejection distances to be used. High filling speed, i.e. low flow value in seconds, is preferred as the time for filling is shorter allowing increased production speed.
By adding a binder, which also may act as a lubricating substance, finer particles such as graphite and other alloying substances in the iron-based powder composition can be bound to the surface of the coarser iron or iron-based powder thus preventing segregation in the composition. Such segregation may otherwise lead to varying properties within the compacted part and increased weight scatter between compacted parts.
Apart from the above mentioned characteristics imposed on a high quality lubricant used in the press and sinter technology of metal powder such a lubricant need also render high green strength to the compacted part. Green strength, i.e. the strength of a component before sintering defined and measured according to ISO 3995, is one of the most important physical properties of green parts. The importance of this property increases with increased complexity of the compacted part. Green strength increases with increased compact density and is influenced by type and amount of lubricant admixed to the powder. The type of iron powder used will also influence the green strength, sponge iron powder having more irregular shape, result in higher green strength compared to atomised iron powder despite the fact that higher green density of the compacted component is obtained when using atomised iron powder. Thus, there is a need of providing a lubricant giving high green strength especially to components made from atomised iron-based powder compositions. In order to increase the green strength the compacted body may be heat treated before sintering.
High green strength is required in order to prevent compacted parts from cracking during ejection from the die and prevent them from getting damaged during the handling and transportation between he press and the sintering furnace. Another advantage obtained by high green strength is the possibility of machining the green component prior to sintering which is of course far more easier than machining the sintered component. This advantage is more pronounced the higher the hardness and strength are of the sintered component, making machining of the green component more attractive compared to machining of the sintered component. This will be specially evident in the case of the component being sinter-hardened.
The development within the powder metallurgical field and especially directed to iron-based powder compositions for pressing and sintering has been intensive and to a great extent focused on bringing new and enhanced lubricants improving the powder properties, die lubrication, green density or green strength. However it has been difficult to obtain a lubricating substance improving all of the essential properties as some of them seem to counteract each other. It is therefore a need to obtain such a lubricant or lubricating composition improving all of these essential properties, especially when used in an atomised iron-based powder composition.
The patent application WO 03/031099 to Ramstedt describes a lubricating combination essentially consisting of 10-60% by weight of polyethylene ether and the remainder being an oligomer amide. This combination enhances the green strength of the compacted part.
U.S. Pat. No. 6,605,251 to Vidarsson discloses a polyolefin-based polymer having a weight average molecular weight of 500-10 000 as well as a method for obtaining high green strength of the compacted part by heating the compacted part up to a temperature above the melting point peak of the polyolefin based polymer. It has however been noticed that when using such polyolefines alone as lubricating agents in powder metallurgical compositions a so called stick-slip phenomenon occurs during the ejection of the compacted body from the die. This means that the body tends to stick to the wall of the die during the ejection, instantaneously increasing the ejection force, and when the component slip, the ejection force needed is instantaneously decreased. This will recur at a high frequency causing a creaky noise, vibrations, high stress on the part subjected to ejection and risk of cracking the part. The stick-slip phenomenon is also revealed as a spiny ejection force curve when logging the ejection force as a function of ejected distance.